Octane is one of the most important product qualities for gasoline. Specifically, octane is a measure of the tendency of gasoline to resist auto-igniting when compressed with air in a spark ignition (Otto cycle) engine.

In a spark ignition engine, a mixture of gasoline and air is compressed in the engine's combustion chamber, before being ignited by a high temperature spark. For efficient engine operation, it is important that the gasoline fuel mixture does not ignite before it is fully compressed. However, because compression raises the temperature of the mixture, some hydrocarbons do have a tendency to auto-ignite at the wrong time in the engine cycle. Octane is a measure of a hydrocarbons resistance to doing this.

Higher octane materials are more valuable to a refiner because they allow the refiner to make higher octane gasoline, which is priced at a premium. Also, high octane material can be used to upgrade cheap, low-octane blend stocks and still achieve finished gasoline quality.

Refiners have a number of options for producing octane. The most important is the reformer, which significantly raises the octane of heavy naphtha. An octane boost also comes from isomerizing light naphtha. Other sources of medium to high octane material are the FCC and the alky unit.

High and low octane materials

Certain chemical structures are associated with high and low octane ratings. Generally speaking, aromatics, olefins, and iso-paraffins (branched paraffins) will have higher octane ratings. Straight-chain paraffins will have lower octane ratings. Molecules containing oxygen, such as alcohols and ethers will also have higher octane ratings.

Consequently, some of the higher octane gasoline blend stocks are:

FCC gasoline from the FCC - high in aromatics and olefins

Reformate from the reformer - high in aromatics

Alkylate from the alkylation unit - branched chain paraffin

isomerate from the isomerization unit - branched chain paraffin

Ethanol - an alcohol

MTBE - an ether

Lower octane gasoline blend stocks are:

Hydrocrackate from the hydrocracker - straight chain, saturated paraffins

Light straight run naphtha - saturated paraffins

Heavy naphtha - saturated paraffins and naphthenes

Measurement of octane rating

Octane is measured using an index reflecting the degree to which a fuel will knock under specific operating conditions. The index is defined based on the knocking characteristics of two reference fuels. Iso-octane has a very high resistance to knocking, even at high levels of compression, and is assigned an octane rating of 100. N-heptane has a low resistance to knocking, even at very low levels of compression, and is assigned a rating of 0. Other fuels are assigned an octane rating based on the mixture of Iso-octane and n-heptane that results in knocking at the same compression level as the fuel being considered. For example, a fuel that knocks at the same compression level as a mixture of 75% iso-octane mixed with 25% n-heptane would have an octane rating of 75.

Because knocking behavior varies with engine operating conditions, there are several different octane scales considered. The two most common are RON (Research octane) and MON (motor octane). These are both measured using the same iso-octane and n-heptane based method, but under different engine operating conditions. RON conditions are meant to reflect fairly stable cruising conditions. MON conditions are meant to reflect more challenging conditions such as high speeds or pulling a heavy load. As a rough rule, RON is about 10 points higher than MON, for the same fuel.

In North America there is also a third octane index commonly used that is simply an average of RON and MON. This is called DON. AKI or R+M/2.